Method of heating travelling stock



NOV. 20, 1934. NORTHRUP 1,981,630

METHOD OF HEATING TRAVELTNG STOCK Original Filed Oct. 28, 1926 2 Sheets-Sheet l Nov. 20, 1934. E. F NORTHRUP 1,981,630

METHOD OF HEATING TRAVELING STOCK 1 Original Filed Oct. 28, 1926 2 Sheets-Sheet 2 Patented Nov. 20, 1934 STATE-ii PATENT OFFICE EATING TRAVELLING it??? rorthrup, Erinceton, N. J.,

trothermic Corporation, Ajax Pars,

Ewing .cwnship, N. 5., a corporation of New Jersey 31 Claims.

My invention relates to methods and apparatus for passing current through travelling stock in the form of rods, bars, sheets or any other electrically conducting material. This application is a continuation of my application Serial No. 144,793, for Resistance heating of travelling stock, filed October 28, 1926.

The main purpose of my invention is to obtain uniformity of current passage through contacts electrically in parallel which engage travelling stock, and to accomplish this by inductive or capacitative reactance in series with the contacts and independently of the differences in surface resistance met with during the travel of the material to be heated.

A further purpose is to use inductance or capacity in series with individual contacts for applying current to travelling stock so as to pass correspondingly large currents through the contacts.

A further purpose is to increase the permissible voltage applied to travelling stock by using artificial inductance or capacity in series with the contacts to increase the impedance of the circuit or circuits through the stock.

A further purpose is to resonate the circuit through the stock in order to secure large currents without corresponding generator supply currents.

A further purpose is to form a resonant circuit in part of travelling stock to be heated and in part of contact leads containing reactance and in which the inductance may or may not itself perform the heating function by surrounding the stock.

A further purpose is to maintain uniformity of current flow through different branches of feeding contact circuits by inductances, which at the e time are utilized inductively to heat the purpose is to tune a circuit containresonated paths and capacities in sepaths by inductance which itself surrounds the stock to aid in heating the stock.

A further purpose is to provide a tuned circuit for passage of current parallel paths by an inductive or capacitative reactance common to each of the paths and opposite reactance individual to the separate paths.

A further purpose is to tune circuit containing divided resistance paths and inductive or capacitative reactance in series with these paths by opposite reactance sufficient to balance all of the first reactance and to place this opposite reactance preferably across the generator su cply but permissibly in series with it.

A further purpose is to apply high frequency current to the conductive heating of travelling stock.

I have preferred to illustrate a few varieties only of what I regard as the same form of my invention, selecting a form for illustration which is practical, effective and reliable, and which at the same time clearly demonstrates the princi- 5 ples involved;

The figures are all diagrammatic views, showing slightly variant ways in which my invention may be applied, but all coming within my broad invention.

Figure 1 is a diagrammatic heating circuit in which capacitative reactance is in series with each contact, and inductive reactance in parallel with the source is used for tuning.

Figure 2 corresponds with Figure 1 except that the tuning reactance is in series with the source.

Figure 3 is a conventional illustration of the circuit of Figure 1.

Figure 4 differs fromFigure l in that the tuning reactance inductively heats the stock.

Figure 5 is a diagrammatic heating circuit in which inductive reactance is in series with each contact and capacitative reactance in parallel with the source is used for tuning.

Figure 6 is a conventional illustration of the 35 circuit of Figure 5.

Figure '7 differs from Figure 5 because the inductive reactance is employed to inductively heat the stock, and the tuning reactance is in series with the source.

Figure "la is a fragmentary view of a variation in the circuit of Figure '7.

In Figure 8 the inductive heating from both sides of the circuit is concentrated in the same portion of the stock, rather than being divided as in Figure '7.

Figures 9 and 19 are fragmentary views of tuning reactance placed in the secondary circuit of a transformer. In Figure 9 the primary of the transformer is in series with the source and in Figure 10 it is in parallel with the source.

In the drawings simiiar numerals indicate like parts.

Any sutiable material which is electrically com ducting and which is long enough to be heated while the material is in motion is referred to by me as travelling stock. The stock will usually be magnetic, but my invention may be used to heat it whether it is magnetic or not.

Travelling stock maybe heated by an electric current inductively or conductively or by a combination of inductive and conductive heating. The dividing line between that which is best done inductively and that which is best done conductively is difficult to set, but for any given material it will largely depend upon the temperature, the cross section, and the suitability of the surface at which contact must be made to the stock.

Conductive heating of travelling stock of magnctizable or non-magnetizable material, whether solid or hollow, has been difficult and unsatisfactory in the past, because it has been impracticable to apply all of the urrent required through a single contact, and with a plurality of contacts it has been commercially impracticable to maintain uniformity through the individual contacts. The resistance of the stock has been low, and pitting and arcing-have taken place, current flow has been interrupted or greatly decreased by variation in contact resistance, and

low voltages only have been permissible.

During the movement of travelling stock the electrical contacts in the respective branches are necessarily poor and variant, not only because of mechanical inequalities in the surface of the stock, and of lateral movement of the position of stock interfering with physical engagement, but also because of oxidation films or other impurities on the surface of the stock, which interfere with and vary the electrical conductivity of the surface.

My invention utilizes a plurality of direct contacts in parallel, with provision for obtaining equal distribution of current through the several paths, raising the voltage and maintaining the power factor of the supply current substantially at unity.

Describing in illustration and not in limitation and referring to the drawings:

.fy invention is primarily concerned with the electrical features of the supply circuits connected to the contacts, and therefore I will generally describe the structure shown diagrammatically for supporting and manipulating the stock, and

-; will apply this description to all of the figures in which this structure appears. V

The stock 21, 21 may be solid or hollow, and may be of any desired cross section. In some of the figures I have indicated solid stock as, while in others I show hollow stock but it will be understood that my invention is applicable to either.

The stock is supported and progressed upon any suitable means, here indicated by rollers 21,

' moving in the direction of the arrows. The direction of progression of the stock and the continuity or intermittence of the progression are of course immaterial to my invention.

Electrical contacts to the stock are shown at 22 and 22'. In order to make possible heating of the stock up to the temperature desired for op erations upon it, such as, for example, upsetting for rivet heads and for other mechanical distortion of the material, I desire a large energy in put, and this may best be obtained by the use of a number of contacts larger than three. It will therefore be understood that I do not limit myself to any special number of contacts. In the drawings I suggest this fact by breaking the stock between the contacts.

The contacts 22 and 22' may be rigid or movable as best suits the particular stock being handled and. the amount of current to be passed through each contact.

I have discovered that uniform flow of electric current through any number of contacts can be maintained within any degree of uniformity that may be desired by passing the current through the contacts in parallel, inserting in each of the divided circuits an inductive or capacitative reactance, large as compared with the contact resistance according to the degree of uniformity desired, so that the resistance component of the circuit impedance may be as small a factor as desired and the current flow will be substantially independent of the resistance.

It will be understood that the branches of the circuit in series with the respective contacts may be capacitativeor inductive, and may be both capacitative and inductive in the individual branches provided either predominates to a suih cient extent. Of course, opposite reactance should be used in the external circuit for tuning, and the character of the opposite reactance will depend upon whether the reactance placed in series with the contacts is positive or negative.

Wherever I refer to reactance, without other designation, I intend to indicate either positive or negative reactance, as may be preferred.

In each of the figures I show a conventional source of alternating current G whose terminals are connected, directly or through reactance, to parallel connections 23 and 24, which are respectively connected to the respective contacts 22 or 22.

For the purpose of making clear that either inductive or capacitative reactance may be used in series with the individual contacts, I show each of the individual branches 25 supplied with capacitative reactance 26 in Figures 1, 2 and 4 and with inductive reactance 26 in Figures 5, 7 and 8. Except for the fact that the character of the tuning reactance is dependent upon the positive or negative character of the reactance 26 or 26, it is electrically immaterial whether capacitative or inductive reactance be used in series in the circuits 25.

Because of the reactances 26 and 26, the power actances 26 of Figures 1, 2 and 4 are respectively 1,

tuned by inductive reactances 27, 27 and 27 while the inductive reactances 26' of Figuresfi, 7 and 8 are respectively tuned by capacitative reactances 27 27 and 27 In Figures 1, 4, 5 and 8 I show the tuning reactance in parallel with the current supply. This arrangement is illustrated diagrammatically in Figures 3 and S. From Figures 3 and 6 it will readily be seen that the oscillation circuit'does not include the current source G, so that heavy I currents may be passed through the stock without the necessity of having the generator capable of carrying such currents.

For example, in Figure 3, the resistance R of the circuit is represented largely by the resistance of the stock and the contact resistances. The capacitative reactance Xc, in series with the resistance R, represents the resultant effect of the various reactances 26 connected in two groups in series and placed in parallel within each group as in Figures 1, 2 and 4. The parallel branches of the circuit, each containing resistance and capacitative reactance, are resonated by inductive reactance XL in parallel with the capacitative branches.

In Figure 6 the situation is reversed. The inductive reactance XL, in series with the resistance R, is the resultant effect of the two parallel-connected groups of inductive reactances placed in series. The resistance and inductive reactance are resonated by the capacitative reactance X0.

It will be evident that in Figure 3 the capacity acts as a current transformer while in Figure 6 the inductance acts as a current transformer.

In Figures 2 and 7 resonating reactance is placed in series with the supply. This has the disadvantage that the generator must be designed to carry the oscillation current. Thus in Figure 2 the capacities 26 are tuned by the series inductance 27. In Figure '7 the inductances 26 are tuned by the capacity 2'7 in series.

In Figures 2 and '7 it will be seen that the variations in impedance of the several branches may be substantially limited so that the current is practically equal in each of the circuits 25, notwithstanding variant conditions of engagement of the stock by the contacts.

The rate of heating of the travelling stock will depend partly upon its resistivity, its character, whether magnetic or non-magnetic, its reluctance if magnetic, the conditions of cooling, including the area of cooling surface, the speed of travel and the size and frequency of the current passing through the stock.

In Figures 4, 7 and 8, I show inductance in the circuit usefully applied to inductively heat the travelling stock. If the stock be of magnetic material, as would usually be the case, the inductance will be most desirably employed at a point in the travel of the stock where the temperature is below the point of decalescence. In Figure 4 the inductance 27 used to resonate the circuit, surrounds the stock to inductively heat it. Electrically the circuit of Figure 4 is substantially identical with that of Figure 1 (shown diagrammatically in Figure 3). g

In Figure 7 the inductive reactances 26' in the individual circuits 25 are supplied by coils surrounding the stock and serving to inductively heat it. The coils for each group of contacts are placed adjacent to those contacts, so that the inductive heating will take place at two points along the stock; The power factor is corrected by the capacitative reactance 27 Where combined inductive and conductive heating is used, the inductor coil must of course surround magnetic stock if it is to use this stock as a core. With high frequency the inductor would induce current effectively in the stock even.

if the latter were non-magnetic. In the case of non-magnetic stock, the stock could not of course act as a magnetic core for the inductance. Assuming, as intended in the illustration, that the stock is magnetic in Figure "I, the coils should be placed at a point in the travel of the stock where the temperature has not been raised to decalescence.

In Figure 7a I show a form intended to heat nonmagnetic stock or stock already heated beyond decalescence. The reactances 26 surround the stock to inductively heat it, but, as the stock is non-magnetic, they add relatively little inductance to the circuit. Therefore I add suitable inductive reactances 26 one in series with each contact, to add to rcactances of the individual branches 25.

In Figure 8 I show the various inductances 26' surrounding the stock to inductively heat it, and

placed atone end of the travelling stock as would be desirable in the case of magnetic stock where the final temperature of the stock is above the decalescence point. The circuit of Figure 8 differs from that of Figure 1 in that the power factor correcting capacitatire reactance 27 has been placed in parallel to correspond with the circuit of Figure 5.

Figures i and i"; are inserted for the purpose mainly of indicating that the tuning of the circuit does not require that the inductive or ca pacitative reactance be directly within the circuit either in series or in shunt. The timing reactance is eifectively included in Figure 9 by placing it in the secondary circuit of a transformer 28 whose primary is in series with the generator circuit. In Figure 10 the tuning reactance is effectively shunted across the generator terminals by placing it in the circuit of the secondary of a transformer 28 whose primary is shunted across these terminals.

In Figure 9 the tuning reactance 29 corresponds generally to the reactance 27 of Figure 7, while in Figure 10 the tuning reactance 28 corresponds generally to 27 of Figure 5.

The forms of Figures 9 and 10 have the advantage that the voltage across the tuning reactance may be adjusted to that for which the reactance is best suited.

In the discussion so far it has been assumed that the invention is applicable to all frequencies of current supply, and, so far as the capability of securing substantially uniform flow of current through all the multiple contacts is concerned, this is the case.

However, high frequency is by far the most desirable, not only because of the greater ease and lower cost of power factor correction with higher frequency, but also because of the greater hysteresis heating in magnetic stock with higher frequency and the increased resistance of the stock due to skin effect.

At high frequency the depth of penetration of the current within the stock is quite low and the effective resistance of the stock to both the current induced in the stock and that conduct ed through the stock is greatly increased, so that the RP heating effect is great in a comparatively short length of stock.

In addition to the uniformity of current se cured by my invention, the RI heating effect is increased by resonating the circuit, which provides for high current input. Whether inductance be applied in series with the individual conta cts or as a tuning reactance in series or in parallel with the source of current, it can equally well be used to induce current in the travelling stock.

Ofcourse the advantages of high frequency are offset to some extent by the increased cost of generator construction, and the ultimate fre quency to be used will be determined by the balpossible to determine by formula the actual resistance encountered; and this difficulty increases with increased temperature. The volts consumed in the travelling stock will therefore vary with the temperature and, in magnetic stock, will vary with the frequency.

The greater input provided by my invention makes it possible to heat the stock within a shorter conductive length than would otherwise be the case, saving in space taken and reducing the heat losses from conduction, convection and radiation.

The current is best supplied by a multi-pole rotary alternator whose volt ampere rating must equal the power to be supplied divided by the power factor of the circuit. Consequently raising the power factor reduces the cost of the alternator. J

The subject will be cleared somewhat by assuming a hypothetical case. Let the travelling stock have a cross section of one square centimeter and an average length of stock L between the contact groups of 100 centimeters. The value of the resistivity p will difier in different parts of the cross section and in difierent parts of the length of the rod-due to differences in temperature, but let us assume an average of 100 10 ohm.

Then in the diagram of Figures 3 or 6 the resistance of the length of stock between the contacts will equal ip 1G' =0.01 ohm o i If the stmk absorb l0 kilowatts, the current will then be Ludo amperes, and the voltage necessary to drive this current through the stock will be 10 volts. Let us assume that the individual contact resistance of each of 10 contacts in parallel be 0325 ohm. Then the contact resistance of a set of 10 such contacts in parallel will be 0.0025 ohm, and since there are two such sets of contacts, the total contact resistance will be 0.005 ohm.

The total circuit resistance is then 0.0l+0.'005=0.015 Ohm.

The power factor is If 500 cycles be used the power factor may be corrected economically with static condensers and the stock will have a much higher resistance than with 60 cycles, for example.

Notwithstanding that 1000 amperes pass through the stock, each contact will carry but 100' amperes and each contact will carry this even with wide variation in the contact resistance.

At such a frequency the reactances in series with the contact may be made as large as desired, so the voltage of the generator may be relatively high, say 110 volts.' The large current through P 22000 I. E-F!- m 1000 amperes.

P=RI

ohm.

As will be seen, this invention is suitable for heating stock in rolls, such as might be used for hack saws or hand saws as well as .stock fed through in strips or bars such as might be used for bolt or rivet stock. It is suitable moreover for heating for a great variety of purposes, including annealing of pipes, bars or strips, and tempering.

Where the material is non-magnetic as in the case of copper tubes or tubes being heated above decalescence to prepare for steps of manufacture, the skin effect will cause an increase in the cffective resistance, increasing with increase in the frequency, and where the material is magnetic the additional heating due to this increased effective resistance will be supplemented by hysteresis.

I recognize that my invention may be applied in various ways differing from that illustrated and obtaining all or part of the benefit of my invention without copying the form shown. It is my purpose therefore to include all such uses of my invention which come within its spirit and scope.

Having thus described my invention, what I claim as new and desire to secure by Letters Pattent is:

1. The method of heating travelling stock which consists in passing high frequency alternating current through the stock lengthwise by means of a plurality of contacts to and from the stock at each end of the portion to be heated and in unifying the flow through the contacts by arranging the contacts in parallel and in giving the parallel parts of the circuit added reactance.

2. The method of heating travelling stock which consists in passing high frequency alternating current through the stock lengthwise by means of a plurality of contacts to and from the stock at each end of the portion to be heated and in unifying the flow through the contacts by arranging the contacts in parallel, in giving the parallel parts of the circuit reactance and in increasing the voltage of the supply in view of the added effective reactance.

3. The method of heating travelling stock which consists in passing high frequency current through it longitudinally and also progressively in a direction extending around the stock circumferentially in the same direction to combine conductive and inductive heating.

4. The method of heating travelling stock by alternating current passed directly through the stock which consists in passing high frequency current to and from the stockthrough connecting paths in parallel and in adding to each of the paths reactance high as compared with the contact resistance to form equal impedance through the several paths.

5. The method of heating travelling stock by alternating current which consists in passing high frequency current into and from the stock through parallel contact paths, in-adding reactance high as compared with the contact resistance to each of the paths, equalizing the impedances in the paths and in tuning the circuit through the paths and the stock.

6. The method of increasing the flow of current passing directly through stock and reducing pitting of the stock which consists in supplying the current to the stock through multiple paths containing added reactance and in adding opposite reactance to approximately equal the added reactance,

7. In the art of heating travelling stock by alternating current passed directly through the stock, the novelty which consists in making contact with the stock by a number of paths in parallel through reactances in the several paths to reduce variation in impedance in the parallel paths due to differences in contact resistance.

8. In the art of heating travelling stock by alternating current passed directly through the stock, the novelty which consists in making contact with the stock by a number of paths in parallel through inductances in the several paths to reduce variation in impedance in the parallel paths due to differences in contact resistance.

9. In the art of heating travelling stock by alternating current passed directly through the stock, the novelty which consists in making contact with the stock by a number of paths in parallel through capacities in the several paths to reduce variation in impedance in the parallel,

paths due to differences in contact resistance.

10. In the art of heating travelling stock by alternating current passed directly through the stock, the novelty which consists in making contact with the stock by a number of paths in parallel through reactances in the several paths to reduce variation in impedance in the parallel paths due to differences in contact resistance and in tuning the circuit.

11. In the art of heating travelling stock by alternating current passed directly through the stock, the novelty which consists in supplying multiple-pole rotary generator current to the stock through a number of paths in parallel, each path having added reactance to reduce variation in impedance in the parallel paths due to differences in contact resistance and in shunting opposite reactance across the terminals of the supply to tune the circuit through the paths.

12. In the art of heating travelling stock by alternating current passed directly through the stock, the novelty which consists in leading the current into the stock and from the stock each by parallel paths through added reactances to increase the impedance of the circuit and to reduce the effect of variation of the resistance in the individual paths and in tuning the combined circuit.

13. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of capacities one in series with each contact and inductance for resonating the circuit including the different contacts, the capacities being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance betwen the contacts and the stock.

14. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of inductances one in series with each contact and capacity for resonating the circuit including the different contacts, the inductances being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance between the contacts and the stock.

15. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of capacities one in series with each contact and inductance in parallel with the source of energy for resonating the circuit including the different contacts, the capacities being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance between the contacts and the stock.

16. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of inductances one in series with each contact and capacity in parallel with the source of energy for resonating the circuit including the different contacts, the inductances being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance between the contacts and the stock.

17. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of capacities one in series with each contact and inductance in series with the source of energy for resonating the circuit including the different contacts, the capacities being large enough to insure high current flow through the individual contacts and to make which the current passes through the contacts in parallel, a plurality of inductances one in series with each contact and capacity in series with the source of energy for resonating the circuit including the different contacts, the inductances being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance between the contacts and the stock.

19. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of reactances one in series with each contact, a transformer whose primary is connected in the circuit and inductance across the secondary of the transformer for resonating the circuit including the different contacts, the capacities being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance between the contacts and the stock.

20. In an apparatus for resistance heating of travelling stock by alternating current, means for guiding the stock, a high frequency source of current, a plurality of contacts adapted to apply said current at diiferent parts of the same section of travelling stock, a plurality of contacts adapted to withdraw the current from the travelling stock and reactances arranged so that each contact is in series with a reactance and the reactances tend to equalize the current through the several contacts independently of the cont'act resistance.

21. In an apparatus for resistance heating of travelling stock by alternating current, means for guiding the stock, a high frequency source of current, a plurality of contacts adapted to apply said current at different parts of the same section of travelling stock, a plurality of contacts adapted to withdraw the current from the travelling stock and inductances arranged so that each contact is in series with an inductance and the inductances tend to equalize the current through the several contacts independently of the contact resistance.

22. In an apparatus for resistance heating of travelling stock by alternating current, means for guiding the stock, a high frequency source of current, a plurality of contacts adapted to apply said current at different parts of the same section of travelling stock, a plurality of contacts adapted to withdraw the current from the travelling stock and capacities arranged so that each contact is in series with a capacity and the capacities tend to equalize the current through the several contacts independently of the contact resistance.

23. In an apparatus for resistance heating of travelling stock by alternating current, means for guiding the stock, a source of high frequency current, a plurality of parallel paths of connection of current to and from the stock and reactance in each path, high as compared with the contact resistance to make the latter a small part only of the impedance.

24. In an apparatus for resistance heating of travelling stock by alternating current, means for guiding the stock, a source of high frequency current, a plurality of parallel paths of connection of current to and from the stock, added reactance in each path, high as compared with the contact resistance to make the latter a small part only of the impedance in each path and opposite reactance to raise the power factor of the supply circuit.

25. In an apparatus for passing alternating current through travelling stock for the purpose of heating, two sets of a plurality of contacts each engaging the travelling stock to pass current through it, a plurality of reactances, one connected to each contact, the reactances and their contacts of each set being connected in parallel, and a source of alternating current connected in series with the two sets.

26. In an apparatus for passing alternating current through travelling-stock for the purpose of heating, two sets of a plurality of contacts each engaging the travelling stock to pass current through it, a plurality of reactances, one connected to each contact, the reactances and their contacts of each set being connected in parallel, a source of.altcrnating current connected in series with the two sets and means for correcting the power factor of the source of supply.

27. In apparatus for heating travelling stock by alternating current, a contact circuit passing current through the stock lengthwise in combination with an inductive circuit supplied from the same source of energy coincidentally passing spirally about, and heating the same stock.

28. In an apparatus for resistance heatingof travelling stock by alternating current passed through it, a source of alternating current, a plurality of contacts at each of spaced parts of the length of the stock engaging the stock, a plurality of inductance coils surrounding the stock, one in series with each of the contacts and connections placing these contacts with their coils in parallel with the source of current supply for inlet and outlet of current to and from the stock.

29. In a heating system for travelling stock, a plurality of inlet and plurality of outlet contacts with the stock, a source of alternating current supply for passing current through these contacts and the stock and an inductance in series with each contact, the inductance surrounding the stock.

30. In a heater for travelling stock, a source of electrical energy, a plurality of contacts adapted to engage the stock to pass current through a portion of the length of the stock, connections by which the current passes through the contacts in parallel, a plurality of capacities one in series with each contact and inductance surrounding the stock for resonating the circuit and for inductively heating the stock, the capacities being large enough to insure high current flow through the individual contacts and to make negligible inequalities of resistance between the contacts and the stock, the inductance being in series with the source of energy.

31. In a heating system for travelling stock, a plurality of inlet and plurality of outlet contacts with the stock, a source of alternating currentsupply for passing current through these contacts and the stock, reactance in series with each contact, a transformer in the circuit and opposite reactance in the circuit of the secondary of the transformer to tune the. heating circuit.

EDWIN FITCH NOR'I'HRUP. 

